This invention is directed to glass compositions for use as cathode ray tube faceplates and, in particular, as faceplates for color television picture tubes.
Whereas the glass faceplate for a cathode ray tube of the type employed in color television picture tubes must exhibit transparency, a high degree of x-radiation absorption, and excellent resistance to discoloration caused by the bombardment of high velocity electrons thereupon and exposure to x-radiation, the process for fabricating cathode ray tubes dictates a matrix of physical properties which the glass should also satisfy. To illustrate:
The faceplate glass ought to demonstrate a coefficient of thermal expansion closely approximating that of glasses customarily used in the manufacture of cathode ray tubes, viz., greater than 97 but less than 100.times.10.sup.-7 /.degree.C. over the temperature range of 0.degree.-300.degree. C., in order to achieve satisfactory compatibility with those glasses and with the metal components which are sealed therein.
To minimize thermal distortion of the glass components during assembly of the tube, the glass should exhibit an annealing point of at least about 500.degree. C. and a strain point of at least about 460.degree. C.
To insure proper operation of the tube, the glass must manifest an electrical resistivity, expressed in terms of Log .rho., greater than 9 at 250.degree. C. and greater than 7 at 350.degree. C.
Where electric melting of the glass is envisioned, easily reducible metal oxides, especially PbO, will most preferably be essentially absent from the glass. It is believed that the inclusion of readily reducible metal oxides in the faceplate composition renders the glass susceptible to discoloration resulting from electron bombardment. Furthermore, whereas As.sub.2 O.sub.3 is a very effective fining agent, because of its ready reducibility, a combination of Sb.sub.2 O.sub.3 and As.sub.2 O.sub.3 will be utilized for that purpose, with the level of As.sub.2 O.sub.3 being held below 0.25% by weight.
MgO will also preferably be essentially absent from the glass. Although the mechanism therefor is not fully comprehended, the inclusion of MgO appears to render the glass more susceptible to devitrification.
Although fluorine has often functioned as a flux to assist in the melting process because its presence does not significantly affect the electrical resistivity or the thermal expansion of the glass (as do the alkali metals), volatilization of fluorine during melting of the glass batch creates a severe air pollution problem. Moreover, the presence of fluorine intensifies corrosion of the molds employed in forming the faceplates. Therefore, various modifications in batch materials have been explored to eliminate fluorine therefrom while achieving similar beneficial fluxing effects without sacrifice of electrical properties.
Finally, for ease in melting and forming, the glass should desirably exhibit as low a liquidus temperature as possible and, in order to form faceplates, should demonstrate a viscosity at the liquidus of at least 100,000 poises. The liquidus has been generally defined in the glass art as that temperature at which crystals begin to form as a glass melt is cooled. Hence, the onset of devitrification commences at the liquidus temperature. Accordingly, a low liquidus permits glass forming to be carried out at lower temperatures without fear of devitrification.
Glasses free from PbO and fluorine which have been used commercially in the production of cathode ray tube faceplates have customarily demonstrated internal liquidi in excess of 850.degree. C. and, frequently, at temperatures in the vicinity of 900.degree. C. It would, however, be highly desirable to produce glass compositions wherein the internal liquidus temperature would be below 850.degree. C., but in which the remaining physical properties would remain essentially constant.